Wireless communication systems are widely known in which base stations (BSs) provide “cells” and communicate with subscriber stations within range of the BSs. In LTE for example, the base stations are generally called eNBs or eNBs and the subscriber stations are called user equipments or UEs.
As an embodiment of the present invention will be described later with respect to LTE, it may be worth briefly outlining some relevant aspects of LTE network topology.
The network topology in LTE is illustrated in FIG. 1. As can be seen, each UE 10 connects over a wireless link via a Uu interface to an eNB 20. It should be noted that various types of eNB are possible having differing transmit powers and therefore providing coverage areas (cells) of differing sizes. Multiple eNBs deployed in a given geographical area constitute a wireless network called the E-UTRAN (and henceforth generally referred to simply as “the network”).
Each eNB 20 in turn is connected by a (usually) wired link using an interface called S1 to higher-level or “core network” entities, including a Serving Gateway (S-GW) 40, and a Mobility Management Entity (MME) 30 for managing the system and sending control signalling to other nodes, particularly eNBs, in the network. In addition (not shown), a Packet Data Network (PDN) Gateway (P-GW) 50 is present, separately or combined with the S-GW, to exchange data packets with any packet data network including the Internet. As shown in FIG. 1, the eNBs 20 communicate among themselves by a (usually) wireless link, using an X2 interface for mutual co-ordination, for example when handing over a UE 10 from one eNB to another.
In such a system, each BS divides its available frequency and time resources in a given cell, into individual resource allocations for the user equipments which it serves, in other words those UEs which have a connection with the BS. The user equipments are generally mobile and therefore may move among the cells, prompting a need for handovers of radio communication links between the base stations of adjacent cells. RRC, or Radio Resource Control, is responsible among other things for signalling related to connection management and handovers to other base stations. A user equipment may be in range of (i.e. able to detect signals from) several cells at the same time, but in the simplest case it communicates with one “serving” cell or “primary” cell. A wireless communication system, and the cells within it, may be operated in FDD (Frequency Division Duplex) or TDD (Time Division Duplex) mode.
FIG. 2 shows the basic units used for resource allocation in the LTE system. Resources in the system have both a time dimension and a frequency dimension. Time in the system is divided in units of a symbol time or “slot” (where a “slot” has typically a duration of seven symbol times), as indicated in FIG. 2. Two successive slots form a “subframe” and (in this example) ten subframes form a “frame”. The frequency bandwidth available in the system is divided into a number of sub-carriers.
The resources available for use by specific UEs are allocated by a scheduling function at the eNB. Such scheduling is usually determined separately for each subframe; in other words the resource allocation of a UE may vary from one subframe to the next. Resources are allocated to UEs both for downlink (DL) and uplink (UL) transmission. UEs which have established a connection with the eNB are synchronized with the eNB and configured with a suitable timing advance (if necessary), so that their allocated downlink and uplink resources can be fully “orthogonal” (non-interfering) with those of other UEs.
In LTE, several channels for data and control signalling are defined at various levels of abstraction within the system.
FIG. 3 shows some of the channels defined in LTE at each of a logical level, transport layer level and physical layer level, and the mappings between them.
At the physical layer level, on the downlink, each eNB broadcasts a number of channels and signals to all UEs within range, whether or not the UE is currently being served by that cell. Of particular interest for present purposes, these include a Physical Broadcast Channel PBCH as shown in FIG. 3. PBCH carries a so-called Master Information Block (MIB), which gives, to any UEs within range of the signal, basic information as described below. Primary and Secondary Synchronization Signals (PSS/SSS) are also broadcast to all devices within range; these carry a physical layer identity and physical layer cell identity group for identifying the cell.
User data as well as System Information Blocks (SIBs) are contained in a transport channel DL-SCH, carried on the Physical Downlink Shared Channel (PDSCH). There are various control channels on the downlink, which carry signalling for various purposes; in particular the Physical Downlink Control Channel, PDCCH, is used to carry, for example, scheduling information from a base station (called eNB in LTE) to individual UEs being served by that base station. The PDCCH is located in the first OFDM symbols of a slot.
Meanwhile, on the uplink, there is a Physical Random Access Channel PRACH which is used to gain initial access to the network, as explained in more detail below. User data and also some signalling data is carried on the Physical Uplink Shared Channel (PUSCH), and control channels include a Physical Uplink Control Channel, PUCCH, used to carry signalling from UEs including channel quality indication (CQI) reports and scheduling requests.
Since the above mentioned MIB and SIBs are important for the invention to be described, some further details will be given here.
The MIB includes some of the basic information which the UE needs to join the network, including system bandwidth, number of transmit antenna ports, and system frame number. Reading the MIB enables the UE to receive and decode the SIBs referred to earlier. With respect to SIBs, the term “receive” henceforth also implies “decode”.
The SIBs differ in their information content and are numbered SIB1, SIB2, and so forth. SIB1 contains cell-access related parameters and information on the scheduling of other SIBs. Thus, SIB1 has to be received by a device before it can decode other SIBs such as SIB2. SIB2 contains information including random access channel RACH parameters, referred to below. Currently, SIBs are defined up to SIB14, although not all SIBs need to be received in order for a UE to access the network. For example, SIB10 and SIB11 relate to an Earthquake and Tsunami Warning System. SIB14 is intended for use with so-called Enhanced Access Barring, EAB, which has application particularly to MTC devices (see below).
For network access, generally SIB1 and SIB2 are the most important, in other words, at a minimum, a UE must normally decode SIB1 and SIB2, in that order, in order to communicate with the eNB. Recently, the present applicant proposed a reduced version of SIB2 called SIB2M, intended for MTC devices (see below), such that reception of SIB1 and SIB2M may suffice for MTC devices to join the network, although SIB2 will still be transmitted for other devices. In the special case of MTC devices subject to EAB, SIB14 is also important.
FIG. 4 illustrates the timings of MIB and SIBs in LTE. As can be seen from FIG. 4, the MIB is broadcast relatively frequently, being transmitted four times in each frame. The SIBs, which unlike MIB are transmitted on PDSCH, occur less frequency. The most essential SIB1 is repeated four times in every other frame, whilst SIB2 and further SIBs typically occur less frequently still. The SIBs are repeated to increase the chance of their being correctly received by a UE, since otherwise, the UE may have to wait an appreciable length of time for the next transmission. This can be a problem particularly for devices at a cell edge or in a coverage hole where reception is poor.
The Physical Random Access Channel PRACH, referred to in connection with FIG. 3, will now be explained since it is also important for the invention to be described. As already mentioned, UEs which have obtained timing synchronization with the network will be scheduled with uplink resources which are orthogonal to those assigned to other UEs. PRACH is used to carry the Random Access Channel (RACH) for accessing the network if the UE does not have any allocated uplink transmission resource. Thus, initiation by the UE of the transport channel RACH implies use of the corresponding physical channel PRACH, and henceforth the two terms RACH and PRACH will be used interchangeably to some extent.
Thus, RACH is provided to enable UEs to transmit signals in the uplink without having any dedicated resources available, such that more than one terminal can transmit in the same PRACH resources simultaneously. The term “Random Access” is used because (except in the case of contention-free RACH, described below) the identity of the UE (or UEs) using the resources at any given time is not known in advance by the network (incidentally, in this specification the terms “system” and “network” are used interchangeably). So-called “signatures” (see below) are employed by the UEs to allow the eNB to distinguish between different sources of transmission. Unlike the RACH in WCDMA for example, the LTE RACH is not designed to carry any user data, although the choice of signature can indicate other information such as the intended size of a subsequent message (see below).
Situations where the RACH process is used include:                Initial access from RRC_IDLE        RRC connection re-establishment        Handover        DL data arrival in RRC_CONNECTED (when non-synchronised)        UL data arrival in RRC_CONNECTED (when non-synchronised, or no SR resources are available)        Positioning (based on Timing Advance)        
RACH can be used by the UEs in either of contention-based and contention-free modes.
In contention-based access, UEs select any signature at random, at the risk of “collision” at the eNB if two or more UEs accidentally select the same signature. Contention-free access avoids collision, by the eNB informing each UE which signature it may use (and thus implying that the UE is already connected to the network). Contention free RACH is only applicable for handover, DL data arrival and positioning.
Referring to FIGS. 5 and 6, the Physical Random Access Channel PRACH typically operates as follows:—
(i) The network, represented in FIGS. 5 and 6 by an eNB 20, informs each UE of the signature to be used for contention-free access, as indicated by “Message 0” in FIG. 5. Periodically, the eNB transmits the broadcast channel PBCH mentioned above, which can be received by all UEs within range (whether or not they are connected to the eNB). The PBCH (not shown in FIGS. 5 and 6) is transmitted once per frame, and is repeated four times (i.e. a complete set of repetitions spans four frames). The PBCH includes the MIB as already mentioned.
The UE 10 receives PBCH for the cell of interest. The information in the PBCH allows the UE to receive further SIBs, in particular SIB1 and SIB2 which are contained in PDSCH.
(ii) As already mentioned, PRACH related parameters are contained in SIB2, including:                time/frequency resources available for PRACH        signatures available for contention-based RACH (up to 64)        signatures corresponding to small and large message sizes.        
The signatures each have a numerical index and the available signatures are indicated by use of a number, with all signatures identified by indices up to this number being available for contention-based access.
(iii) The next step differs depending on whether contention-based access or contention-free access is being attempted.
For contention-based access the UE selects, at random, a PRACH preamble signature according to those available for contention based access and the intended message size. The term “signature” is generally used to refer to characteristics of the particular PRACH preamble transmission. In LTE this corresponds to the preamble sequence. More generally, the signature may include the time domain resources and/or the frequency domain resources, which can include not only the location of such resources in time (symbol no.) and frequency (subcarrier) but also their extent in time and frequency (e.g., number of symbols, number of subcarriers). Henceforth the terms “preamble”, “preamble sequence”, “preamble signature” and “signature” will be used interchangeably, unless the context demands otherwise.
In the case of contention-free access, the UE employs the PRACH preamble signature which has previously been assigned to it via Message 0.
(iv) The UE 10 transmits the PRACH preamble (labelled “Message 1” in FIGS. 5 and 6, also labelled (1) in FIG. 6) on the uplink of the serving cell. The eNB 20 receives Message 1 and estimates the transmission timing of the UE. The PRACH preamble transmitted by a UE, having a certain signature, results in a distinctive waveform being received by the eNB, and the eNB makes a decision about which signature(s) the waveform corresponds to, by correlating it with all the possible transmitted signatures.
(v) The UE 10 monitors a specified downlink channel for a response from the network (in other words from the eNB). In response to the UE's transmission of Message 1, the UE 10 receives a Random Access Response or RAR (“Message 2” in FIGS. 5 and 6, also labelled (2) in FIG. 6) from the network. This contains an UL grant for transmission on PUSCH and a Timing Advance (TA) command for the UE to adjust its transmission timing. FIG. 6 shows the details of the RAR, showing the Timing Advance and UL Grant fields as well as (in the case of contention-based access) a Temporary Cell Radio Network Temporary Identifier (T-CRNTI) field, by which the RAR informs the UE of an identifier which it should use in its uplink communications following RACH. In contention-free access, the UE can be assumed already to have a C-RNTI.
(vi) For contention-based access, in response to receiving Message 2 from the network, the UE 10 transmits on PUSCH (“Message 3” in FIGS. 5 and 6, labelled (3) in FIG. 6) using the UL grant and TA information contained in Message 2. Message 3 includes a RRC Connection Request as shown in FIG. 6, and is the “subsequent message” whose intended size can be indicated by the choice of preamble signature as mentioned above.
In the case of contention-based access, there is the chance that the same preamble sequence may coincidentally be chosen by another UE also initiating random access. A contention resolution message (not shown) may be sent from eNB 20 in the event that the eNB 20 received the same preamble signature simultaneously from more than one UE, and more than one of these UEs transmitted Message 3. If the UE does not receive any response from the eNB, the UE selects a new signature and sends a new transmission in a RACH sub-frame after a random back-off time.
(vii) Further steps, shown in FIG. 6, include a RRC Connection Setup (labelled (4) in FIG. 6) by which the eNB responds to the RRC Connection Request, and a reply from the UE in the form of a RRC Connection Setup Complete message as labelled (5) in FIG. 6.
FIGS. 5 and 6 show the signalling sequence in a simplified form. There is also signalling between the eNB and MME 30 and the S-GW of FIG. 1. FIG. 7 is a more comprehensive signalling diagram for the case of contention-free access, including this higher-level signalling. As is apparent from FIGS. 5-7, the network access procedure in LTE is considerably involved and may occupy a significant amount of time, particularly if the initial steps are delayed by difficulty in receiving the SIBs referred to earlier, and/or if there is a need for contention resolution in contention-based access. Although it is possible to repeat transmission of SIBs to assist reception, this only extends the time taken to complete network access. Moreover, the power consumption involved may be significant for low-power devices.
Meanwhile, the advent of machine-to-machine communications (M2M) between e.g. smart meters in homes and an LTE network creates a large number of deployed devices (so-called Machine Type Communication or MTC devices) which must be low cost, low power, are generally deployed statically and have low-rate, possibly periodic data transmissions with potentially long gaps. This scenario is also referred to as Small Data Transmission.
It is therefore desirable to design signalling which is more efficient than existing LTE signalling by being targeted at the Small Data Transmission scenario, particularly but not exclusively with respect to MTC devices. In particular there is a need to support frequent transmission of small amounts of data efficiently with minimal network impact (e.g. signalling overhead, network and radio resources, and delay for resource reallocation). It would also be desirable to facilitate access to the network for devices (such as MTC devices) at a cell edge or coverage hole.